As is known, formed bodies based on expanded styrene po lymers are obtainable by molding, in closed and not gas-tight molds, polymer particles containing gaseous or liquid expanding agents, by operating at a temperature higher than the boiling point of the expanding agent and higher than the softening point of the polymeric material.
According to a usually followed operative mode, the polymer particles containing the expanding agent are first heated (pre-expansion) in a mold, either closed or open, up to a prefixed apparent density and then, after a proper ageing time (about 24 hours), are further expanded by means of heat (molding) in a gas-untight and pressure-resisting mold.
As a consequence of such heating, the particles sinter, owing to the narrow space available, thus forming a body having the shape and the dimensions of the cavity of the utilized mold.
After molding, the formed body is allowed to cool in the mold for a period of time which is long enough to limit the deformation of the formed body once withdrawn from the mold.
As the expanded plastic material is an excellent heat insulating material, relatively long dwell times in the mold are requied for cooling the formed body.
Furthermore, the expandable styrene polymers which contain a self-extinguishing organic halogen-containing compound exhibit a high shrinkage of the molded articles, what prevents from obtaining molded bodies with pre-established dimensions. A further disadvantage of these polymers resides in the fact of a high loss of expanding agent during the pre-expansion operation, so that the obtained molded bodies exhibit a low sintering degree due to the low concentration of the residual expanding agent and, consequently, to the low molding pressure.
Furthermore, the expanded particles have a cellular structure with very fine cells abd the cells' walls are perforated or open, thus not permitting the attainment of high pressure during molding.
In U.S. Pat. No. 4,029,614, there is disclosed expandable styrene polymers, containing a foaming agent, a self-extinguishing organic halogen-containing compound and from 0.0001 to 1% by weight of the styrene polymer, of an amine free from oxy-alkylated groups and having a general formula NR.sub.1 R.sub.2 R.sub.3, wherein R.sub.1 is an aliphatic or a cycloaliphatic hydrocarbon radical of from 4 to 20 carbon atoms and R.sub.2 is hydrogen or an aliphatic or a cycloaliphatic hydrocarbon radical of from 1 to 15 carbon atoms and R.sub.3 is hydrogen or an aliphatic or aromatic hydrocarbon radical of from 1 to 15 carbon atoms, with the prevision, that when both R.sub.2 and R.sub.3 are hydrogen, then R.sub.1 has 6 to 20 carbon atoms.
In U.S. Pat. No. 4,192,922 there is disclosed foamable particles of a styrene polymer containing an expanding agent, a flameproofing organic halogen compound and from 0.0001 to 0.1 by weight, relative to the polymer, of a 2,4-diamino-1,3,5-triazine derivative.
Tests carried out by the Applicant have evidenced that none of the above-mentioned compounds gives entirely satisfactory results, particularly as regards sintering and shrinkage of the manufactured articles.